This invention relates to the control of an internal combustion engine having intake and exhaust camshafts that are independently and continuously variable, and more particularly to a method of estimating the volumetric efficiency of the engine for purposes of engine fuel control.
Accurate control of engine air/fuel ratio requires knowledge of the mass air flow entering the engine cylinders in each combustion cycle. Ordinarily, this can be accurately determined through a speed-density calculation based on the measured engine speed (ES) and intake manifold temperature (MAT) and absolute pressure (MAP), taking into account various factors including the volumetric efficiency of the engine. The volumetric efficiency, in turn, can be estimated based on engine speed ES and a pressure ratio (PR) between the intake and exhaust manifolds of the engine; see, for example, the U.S. Pat. No. 5,714,683, issued on Feb. 3, 1998, and incorporated herein by reference.
The above-described process must be adjusted for engines equipped with cam phasing mechanisms because varying the phase of an intake or exhaust camshaft (relative to the crankshaft) changes the breathing characteristics, and therefore the volumetric efficiency, of the engine. In the above-mentioned U.S. Pat. No. 5,714,683, this is achieved in the case of single or dual-equal cam phasing by computing a cam-phase compensated mass airflow as a function of the cam phase angle using a second-order equation to approximate a nominal mass airflow vs. cam phase relationship, and then adjusting the nominal mass airflow for pressure and temperature effects. In that approach, the coefficients of the second-order equation are determined by table-look-up based on engine speed ES and the pressure ratio PR, with the table values being determined by bench calibration for various combinations of cam phase angle, engine speed ES and pressure ratio PR. Theoretically, a similar approach could be used to determine volumetric efficiency, from which mass airflow could be computed as mentioned in the preceding paragraph. However, such an approach becomes highly impractical for engines having independently controlled variable intake and exhaust cam phasing mechanisms since the number of required look-up tables (and consequently, the required calibration effort) increases dramatically. Accordingly, what is needed is a more practical and efficient method of accurately estimating the volumetric efficiency of an engine having dual independent cam phase variation.
The present invention is directed to an improved method of estimating the volumetric efficiency of an internal combustion engine having independent intake and exhaust cam phase variation, the method providing a significant reduction in stored data requirements and calibration effort, compared to known methods. According to the invention, the effects of intake and exhaust cam phase variation are de-coupled, and a nominal or base estimate of the volumetric efficiency is compensated for dual cam phase variation in a process involving two successive stages: an intake stage, and an exhaust stage. The intake stage compensates for the effects of intake cam variation, using the base volumetric efficiency estimate as a starting point; and the exhaust stage compensates for the effects of exhaust cam variation, using the output of the intake stage as a starting point. The volumetric efficiency so compensated is then used to accurately compute the mass intake airflow for engine control purposes.